Over 600 million RFID (radio frequency identification) tags are estimated to have been delivered in 2005. Applications of such devices range from identification and access control to counterfeit prevention and logistics. Such tags may be either active or passive, active RFID tags being powered by a permanent power supply (for example provided by a battery) while passive RFID tags are powered via an RF link and therefore only operate when in communication with a reader. Supply chain monitoring is a large market for active tags equipped with a sensor and a memory for storing measured data. For example, active tags having a temperature sensor can be configured to record the actual temperature of frozen food or other goods that need to be cooled or maintained within a preferred temperature range. Depending on the allowed ‘thermal budget’, i.e. an indication of the integrated temperature during storage and transport, the shelf-life of an associated product can be calculated on an individual or batch basis. Moreover, such active tags can indicate if certain limits such as a maximum or minimum temperature have been reached and, if so, whether the product must be discarded. Besides temperature, other parameters such as pH and gas composition, which may also be used for determining the lifetime of a product, could usefully be incorporated into such sensors. Problems related to measuring such other parameters, however, include those of integration and miniaturisation.
Electrochemical sensors, such as pH sensors based on the ISFET (Ion-Selective Field Effect Transistor) principle, require a reference electrode to define the potential of the analyte. The reference electrode maintains the analyte (electrolyte) potential at a fixed value irrespective of the analyte composition. The reference electrode is therefore an essential component of a pH sensor.
A standard reference electrode is, by definition, based on a hydrogen electrode, composed of a platinized (i.e. platinum black coated) platinum sheet immersed in an acid electrolyte solution through which hydrogen gas is bubbled. This standard electrode is clearly not practical for large scale commercial use or for miniaturisation. More practical types of standard electrodes such as those based on Ag/AgCl reference electrodes are frequently used instead. An Ag/AgCl based reference electrode structure typically comprises a chlorinated silver wire (Ag/AgCl) in contact with a well defined reference electrolyte, such as an aqueous KCl solution (typically 3M KCl). Galvanic contact to the analyte is generally established via a diaphragm, which may be in the form of a porous frit made from an inert material such as a glass or ceramic. During operation, the electrolyte must continuously flow out of the reference electrode into the analyte. Other types of reference electrodes include those based on mercury and mercury chloride (also known as a calomel) or thallium and thallium chloride Tl/TlCl electrodes may be used for specific applications such as elevated temperatures. The general principle is, however, the same as for an Ag/AgCl electrode, in particular through the use of liquid reference electrolyte and contact via a diaphragm.
When considering the use of reference electrodes for miniature pH sensors, there are several disadvantages of the above standard types of reference electrodes, including a large form factor, requiring at least several cubic mm to provide sufficient volume for the reference electrode structure, relatively high cost, and the need for the electrolyte to be refilled at regular intervals. These disadvantages make the use of standard reference electrodes for miniaturized chemical sensors, for example for integration into RFID tags, difficult or impossible. Depletion of the reference electrolyte is, however, a relatively minor issue for sensors with RFID applications due to their limited service life.
A. Simonis et al., in Electrochimica Acta 51(2005) 930-937 disclose a planar reference electrode structure having a gel-based reference electrolyte formed on a silicon substrate attached to a PCB (printed circuit board). An Ag/AgCl layer is formed on an oxidised surface of the silicon substrate, and a gel electrolyte of either agar or pHEMA (poly-(2-hydroxyethyl methacrylate)) with KCl provides a defined ion concentration at the interface to the Ag/AgCl electrode, in a similar way to a 3M KCl solution used in conventional reference electrodes. A diffusion barrier layer of PVC and nafion or of cellulose nitrate is provided on the gel electrolyte to limit out-diffusion of KCl from the electrolyte, in order to increase the lifetime of the device. If too much KCl is removed from the gel the KCl concentration at the Ag/AgCl electrode interface changes, causing the reference potential to change. The maximum service life is then reached, as pH measurements are then inaccurate. The diffusion barrier also mechanically protects the underlying gel electrolyte.
The type of reference electrode structure described by A. Simonis et al. is intended to be integrated with an ISFET type sensor together with other electronic components such as amplifiers, microcontrollers, memory, RF units etc. A disadvantage of this approach, however, is that the reference electrode structure needs to have a minimum area of at least several mm2, in order to form a low ohmic contact and to maintain a minimum ion reservoir volume for proper operation. If the volume of the gel reservoir is too small, the ions become depleted too quickly and the device will have only a very short service life. Any practical solution, where a service life of the order of hundreds or thousands of hours is required, would typically require far too much chip area to be commercially viable, particularly for use as part of an RFID device, in which the silicon area is generally less than 1 mm2 and the cost of such devices needs to be minimal.